There are a great variety of commercially available machines for mowing lawns and related landscape and agricultural purposes. A popular kind of machine comprises a frame, a wheel at each of the four corners, a rotary mower cutter deck more or less centrally located between the wheels, and an operator seat proximate the rear two wheels. Typically, the two rear wheels are driven to move the mower, and they support the preponderance of the machine and operator weight.
Different means are used for steering such vehicles. Preferred is the so-called transmission steer machine, where the mower has two freely pivotable front wheels; and directional control, or steering, is obtained by applying differential power to the two independently driven rear wheels.
Rider mowing machines must give good results for a variety of terrains. It is more difficult in general to mow slopes and undulating surfaces. In particular, when traversing across a slope, there tends to be an imbalance in downward force between the rear wheels, from which directional control can be lost, due to downhill shifting of the line of action of the machine center of gravity. Prior art machines, where the wheels are rigidly mounted at the corners of the vehicle, are particularly prone to this effect. This is because, as the machine moves forward, when one front wheel is lifted upward by a small undulation or obstacle, the load at the rear wheel directly behind is lessened. When the front wheel lifted is the uphill wheel, the already low downward force on the uphill rear wheel can become so low that no traction is maintained. As a result, the vehicle will tend to turn and run downhill.
There have been commercial machines in which the front axle is mounted off a center pivot. See for example, U.S. Pat. No. 3,686,840 to Root and U.S. Pat. No. 4,077,191 to Pittinger, Sr. et al. If such design is applied to transmission steerable machines, the above mentioned rear-wheel-lifting effect is avoided.
However, it also a feature of various prior art riding mowing machines that the front wheels will be dropped down from the primary elevation of the axle, whether pivoted or fixed. They will be attached to struts which extend downwardly from the outermost ends of the axle, for the obvious reason of permitting the use of smaller wheels while providing clearance for obstructions or a suspended rotary mower deck. While this configuration serves the intended purpose, when front wheels horizontally hit obstacles, the axle and the pivot point are subjected to very heavy bending moments which can and do cause failure. This necessitates either avoiding the pivot axle type of construction or providing unduly heavy and therefore undesirable pivots and structure.
Thus, there is a need for improved front suspension systems which better maintain contact of wheels with the surface of the earth and enable better control. It is in general a desire to keep overall machine length short, and to keep the center of gravity low; and thus any improvement ought be compatible.